Short answer: The title of this thread could be, "Timing the market - 2016"
Longer answer: If you think the market is on sale then go for it. The Schiller PE ratio stands at about 24 right now which to me, is more or less as irrationally exuberant as the low 27s we were seeing when the market was hitting all time highs in 2015.
Front loading your 401k is equivalent to calling the bottom. There is nothing wrong with calling a bottom, but it should probably be backed up by something more than the assumption that the market is "down now and will be better at the end of the year."
Take 2008 for example. On this day in 2008, the S&P was around 1330, down from a high of about 1560 the prior year. By the end of 2008, the index was hovering around 900. A theoretical person maxing out their 401k who front-loaded at the start of 2008 would have cheated themselves out of about $4K in purchasing at or near the record bottom.
Of course I cherry-picked one of the worst years in modern history but the point still stands. Not all calendar years are up years. Most are, but then again, most 12 month periods probably are as well so you could make a case for loading in any month of the year.
Real Answer: Depending when a person plans to access their retirement accounts, it probably doesn't matter.